1. Field of the Invention
The present invention relates to an inverter apparatus having an inverting portion therein, in particular, relates to an inverter apparatus in which a high speed switching element can be used as a main element suitably.
2. Description of Related Art
Conventionally, in use of a switching element, such as a GTO, it is said that the turn-on mechanism starts in a vicinity of the gate thereof and it spreads out and expands to a whole thereof sequentially. The velocity of this spreading out is approximately 0.03-0.1 mm/.mu.s.
A switching current almost rises up during a time period which is less than several .mu.s in a turn-ON time. Therefore, in an initial period of the turn-ON, total current concentrates in the vicinity of the above-mentioned gate to increase an electric power loss per a unit area in the turn-ON region, thereby melting the vicinity of the gate which is destroyed.
For preventing such destruction, conventionally, various technologies are proposed, including a provision of such as so-called a snubber circuit, and as one of them, a minimal ON/OFF time is determined for a gate pulse for the purpose of restricting time sequential change in a switching current.
However, with an IGBT (abbreviation of "insulated gate bipolar transistor") or MOS-FET as one of a high speed switching element, to which the present invention relates, there is defined or established no such the time restriction, such as the minimal ON/OFF time or the like. The reasons of no such the restriction will be studied by referring to FIG. 11. FIG. 11 shows a basic construction of a n-channel type IGBT.
In the n-channel type IGBT shown in the figure, when a positive voltage is applied to a gate electrode G, on a basis of that of an emitter electrode E, the surface concentration of a p layer just under the gate electrode G is reversed to form a n channel ch, thereby conducting current due to electrons in a n minus (n.sup.-) layer.
Further increasing up the gate voltage, and when it comes to be greater than a gate threshold voltage, the electron current flowing through the channel is amplified, and at the same time, since a p plus layer (hereinafter, it is described by "p.sup.+ layer") and a n minus layer (hereinafter, it is described by "n.sup.- layer") are biased in a forward direction, hole injection occurs from the p.sup.+ layer and the resistance value of the n.sup.- layer comes to be small, thereby obtaining an operation of high current density.
As can be seen from the above operation of high current density, it is because, in particular in the IGBT, due to multi-channels being formed with scattered diffusion areas as well as the skin effect, no such current concentration occurs as in the GTO, thereby occurring no destruction in the vicinity of the gate electrode G due to the electric power loss.
As mentioned in the above, no particular attention was paid to the time of the driving signals for the high speed switching elements, such as the IGBT or the MOS-FET, within a conventional inverter apparatus, therefore, ON/OFF signals obtained as calculation results from a controller portion are outputted irregularly (without restriction) as the driving signals for them.
However, because of development of high speed micro-computers in recent years, even the driving logical ON/OFF signals, being shorter than 0.1 .mu.sec in the time duration thereof, can be outputted from the controller portion, into which such the high speed micro-computer is applied.
Therefore, in particular, with an OFF pulse to the IGBT, if an ON pulse is inputted again when the current does not decrease down sufficiently, the di/dt in the IGBT comes to be abrupt or sharp, thereby causing vibration on the voltage between the collector C and the emitter E thereof (hereinafter, it is described as "Vce"). And, if a fly wheel diode reversibly connected but in parallel (hereinafter, described as "FWD") is inferior in the characteristic, i.e., in the reverse recovery characteristic, there is a problem that the IGBT possibly reaches to effect destruction or breakdown thereof.